In order to achieve the maximum possible voltage handling capability of a metal-semiconductor Schottky barrier contact, the contact should approach its behavior as if it were an infinite parallel plane. Such a theoretical structure would have a uniform electric field at its interface, increasing with applied voltage until it reaches the critical electric field for the onset of impact ionization in the underlying semiconductor. Since it is not possible to have an infinite extent in practice, and finite edges will exist with increased electric field due to curvature of equipotential lines at the contact comer leading to field crowding, the edge of the terminal must be treated in such a way as to make it behave as close to the ideal case as possible. One possible way to accomplish this is by reducing the potential distribution gradients and thus electric field. Another possibility is to alleviate the effect of the increase in electric field.
Various techniques such as doped guard rings, field plates, and mesa etching have been used for edge termination. Doped guard rings are difficult to form in GaN due to the high temperature activation anneal for implanted dopants, and the low diffusion rates of dopant species. Field plates improve the edge termination but do not approach the ideal parallel plane breakdown voltage. Mesa etching removes desirable planarity, and requires careful surface passivation to maintain stability of its effect. Mesa edge termination also falls short of the ideal breakdown voltage.